As described in the book "Industrieelectronik" ("Industrial Electronics"), Springer Verlag, 1973, at page 70, a PI controller in an alternating control circuit of the above-mentioned type comprises an operational amplifier having an output connected to an inverting input via a decoupling component and an RC stage, an output of the second controller being connected via a decoupling diode to the decoupling component at an end thereof opposite the output of the operational amplifier. The first or PI controller functions as a voltage control with alternating current limiting by the second controller. As described in the book, the decoupling component is realized in the form of a diode.
In this conventional alternating control circuit, the voltage control loop is normally closed. If (and only if) the current in the regulated device exceeds a predetermined limit or threshold, the current controller relieves the voltage controller and regulates the current to the permissible maximum. The voltage controller resumes control only upon the current's falling below the threshold valve. A seamless transition between the controllers is possible by means of the decoupling diodes.
A likely difficulty in the above described alternating control circuit involves charge reversal of feedback capacitors. Such capacitors are connected so that they are always charged up to the control voltage of the next-following converter, so that upon an alternation no control discontinuity occurs which is caused by a current surge of a capacitor in the feedback path. In such alternating control circuits, diodes are ineffective as decoupling elements if the signals at the outputs of the respective control amplifiers change polarities. If a faulty state of the regulated device or of the control element leads to an excessively large current value, then the voltage controller can be overdriven and fall out of engagement. While the output voltage of an operational amplifier can be kept within the nominal range in a controller, it would make sense for the protection of the controlled regulated device and, therefore, of the entire system, if an operator were alerted to the existence of an overdrive and if, in addition, the overdrive, would lead to the shutting off or disabling of the regulated device.